The fabrication of robust interfaces between transition metal oxides and non-aqueous electrolytes is one of the great challenges of lithium ion batteries. Atomic layer deposition (ALD) of aluminum tungsten fluoride (AlWxFy) improves the electrochemical stability of LiCoO2. AlWxFy thin films were deposited by combining trimethylaluminum and tungsten hexafluoride. in-situ quartz crystal microbalance and transmission electron microscopy studies show that the films grow in a layer-by-layer fashion and are amorphous nature. Ultrathin AlWxFy coatings (<10 Å) on LiCoO2 significantly enhance stability relative to bare LiCoO2 when cycled to 4.4 V. The coated LiCoO2 exhibited superior rate capability (up to 400 mA/g) and discharge capacities at a current of 400 mA/g were 51% and 92% of the first cycle capacities for the bare and AlWxFy coated materials. These results open new possibilities for designing ultrathin and electrochemically robust coatings of metal fluorides via ALD to enhance the stability of Li-ion electrodes.
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1. A battery comprising:
LiCoO2 cathode core;
a metal fluoride passivation coating conformally coated on the cathode core;
the metal fluoride passivation coating consisting of AlWxFyCz excluding oxygen, the metal fluoride coating comprising W particles uniformly dispersed in an ALF3 matrix; and
wherein x>0, y>0, and z> or equal to 0.
2. The battery of
6. The battery of
7. The battery of
8. The battery of
9. The battery of
10. The battery of
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This application is a divisional of U.S. patent application Ser. No. 14/693,719, filed on Apr. 22, 2015, which claims the benefit of U.S. Provisional Application No. 62/128,944 filed on Mar. 5, 2015, which are hereby incorporated by reference in their entirety.
This invention was made with government support under Contract No. DE-AC02-06CH11357 awarded by the United States Department of Energy to UChicago Argonne, LLC, operator of Argonne National Laboratory. The government has certain rights in the invention.
The present invention generally relates deposited coatings, specifically to atomic layer deposition of metal fluoride coatings.
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.
The fabrication of robust interfaces between electrodes and electrolytes that satisfy the application-dependent, electrochemical demands of various systems is one of the great challenges of interfacial science. In particular, irreversible reactions between non-aqueous electrolytes and metal oxides are often considered to be a rate-limiting step adversely affecting the overall performance of lithium-ion (Li-ion) batteries. Li-Ion batteries, in general, suffer irreversible capacity losses during high voltage operation due, in part, to corrosion of active materials in the acidic electrolyte as well as electrolyte decomposition. Small levels of moisture (˜20-100 ppm) present in battery-grade electrolytes can react with lithium salts (e.g. LiPF6) to generate hydrofluoric acid (HF). HF subsequently attacks the surfaces of electrode materials causing transition metal dissolution and migration to the negative electrode. Furthermore, surface layers can form as a result of electrolyte oxidation at high voltages which impede Li-ion diffusion.
Coating electrode surfaces with metal oxides such as Al2O3, MgO, ZnO, and TiO2 has proven to be effective in mitigating irreversible side reactions. These metal oxides coatings, however, are also susceptible to HF attack and may not be stable over long-term cycling, converting partially to metal fluorides when scavenging HF as reported earlier. Furthermore, byproducts of these reactions generate additional water in the electrolyte again making electrode/electrolyte interfaces unstable. In addition, the high electronegativity of fluorine results in strongly bonded cations relative to oxygen and should be beneficial in limiting unwanted surface reactions. In particular, AlF3-coated positive electrodes (LiCoO2, LiNi1/3Mn1/3Co1/3O2, Li[Li0.19Mn0.57Ni0.16Co0.08]O2, etc.) have demonstrated significant improvements in cycling stability and safety when AlF3 is applied via solution-based routes. The AlF3 layers provide some resistivity to HF attack, but are insulating resulting in decreased cathode performance.
However, wet-chemical processes, as well as standard physical vapor deposition (PVD), have limitations. Non-uniform thicknesses and/or compositional variation of coating layers can arise from the directional nature of deposition (PVD) or by the secondary heating steps required for solution-based routes. As thickness can alter the impact the layer has on the underlying cathode performance, the ability to finely tune and control thickness is important.
One embodiment of the invention relates to a method of preparing a metal fluoride passivation coating on a cathode comprising: performing an atomic layer deposition cycles of TMA at a first deposition temperature between 50° C. and 300° C.; and performing b atomic layer deposition cycles of WF6 at a second deposition temperature between 50° C. and 300° C.
Another embodiment relates to a battery comprising a LiCoO2 cathode core; a metal fluoride passivation coating conformally coated on the cathode core; the metal fluoride passivation coating consisting essentially of AlWxFyCz wherein x and y are greater than 0.
Additional features, advantages, and embodiments of the present disclosure may be set forth from consideration of the following detailed description, drawings, and claims. Moreover, it is to be understood that both the foregoing summary of the present disclosure and the following detailed description are exemplary and intended to provide further explanation without further limiting the scope of the present disclosure claimed.
The foregoing and other objects, aspects, features, and advantages of the disclosure will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:
Atomic layer deposition (ALD) is a modified form of chemical vapor deposition (CVD) that uses the self-limiting nature of specific precursors to deposit films in a layer-by-layer fashion. ALD is particularly well-suited for coating electrode surfaces in that a conformal coating can be applied with precise control of thickness and composition. Previous studies on ALD-coated electrodes have primarily focused on metal oxides such as Al2O3, TiO2, LiAlO2, and LiTaO3 because the ALD chemistry of these oxides is well known. Metal fluoride growth by ALD is complex and challenging, mainly due to the lack of suitable fluorine precursors. For example, HF, a highly aggressive chemical etching agent, has been used to deposit CaF2, ZnF2, and SrF2. More recently, alternative ALD chemistries have been developed such as MgF2 and LiF ALD using either TaF5 or TiF4 as the fluorine precursor for optical applications. However, the substrate temperatures in these cases were 300-400° C.; high enough to degrade battery electrode laminates containing polymeric binders. Another potential limitation of AlF3 for Li-Ion batteries is that it is a wide-bandgap dielectric and hence electrically insulating. Although still promising as a coating, methods to enhance the material's conductivity while maintaining its superb resistance to chemical attack could be advantageous.
U.S. Pat. No. 8,921,799 and pending application published as U.S. Pat. App. Pub. No. 2012/0187305 describe a general method and materials from the method relating to atomic layer deposition of a composite coating. Described herein is a method for and composition and product having ultrathin, amorphous, composite aluminum-tungsten-fluoride (AlWxFy) films on, in one embodiment, LiCoO2 electrodes via ALD using, in one embodiment, trimethyaluminum (TMA) and tungsten hexafluoride (WF6), at 50° C. to 300° C. preferably at 200° C. Such films are created with metal fluoride and do not utilize a metal oxide. There is no oxidant step involved in TMA-WF6 process; TMA reduces tungsten terminated surface in various embodiments, AlWxFyCz is formed where x and y are any non-zero positive number and wherein z can be zero (no carbide), or any positive number. These films are highly conducting, but incorporate AlF3 in the composite. Although LiCoO2 is the most commercially successful cathode material for Li-ion batteries, the practical use of LiCoO2 is limited, in part, to surface reactions involving cobalt dissolution, electrolyte oxidation, as well as structural transformations occurring at potentials higher than ˜4.3 V (vs. Li/Li+). As such, LiCoO2 might be considered as a model system for surface studies on Li-ion cathodes. Ultrathin AlWxFy coatings (˜1 nm) on LiCoO2 are shown to significantly increase stability relative to bare LiCoO2 when cycled up to 4.4 V. These results reveal new possibilities for designing ultrathin and electrochemically robust coatings of metal fluorides via ALD, and potentially other techniques, to enhance the stability of Li-ion electrodes. Using high vapor pressure precursors are beneficial when implemented in role-to-role ALD or spatial ALD.
ALD of AlWxFy was accomplished using alternating exposures of WF6 and TMA. In-situ quartz crystal microbalance (QCM) measurements recorded during alternating, 1 second WF6 and TMA exposures at 200° C. showed a staircase pattern comprised of 160 ng/cm2 steps consistent with layer-by-layer growth (
X-ray photoelectron spectroscopy (XPS) survey scans, performed during depth-profiling of these films deposited on silicon, revealed that they are comprised of W, C, Al, and F. Higher resolution XPS analysis demonstrated that the Al and F are bound as AlF3, and that the W may be present as both metallic W and tungsten carbide (WCx) (See
To evaluate the electrochemical properties of the AlWxFy coating, 5 ALD cycles of TMA-N2-WF6-N2 (˜1 nm) were applied on laminates of LiCoO2. XPS survey scan of coated LiCoO2 confirmed that the AlWxFy films are deposited on the laminates (
Various cycles and supercycle arrangements can be utilized.
In summary, a novel ALD process allowing the deposition of metal fluoride materials has been developed for surface protection of Li-ion battery electrodes. A ˜1 nm AlWxFy film deposited on LiCoO2 electrodes has been shown to dramatically enhance cycle life and rate capability of Li+/LiCoO2 cells. These AlWxFy thin film composites appear to combine the chemical inertness of AlF3 with the high electrical conductivity of a metal. This study suggests new opportunities for ALD and the design of advanced surface structures enabling high capacity lithium ion batteries. Moreover, adjusting the composition of the ALD composites may yield further improvements in performance.
Atomic Layer Deposition: Aluminum fluoride-based composite films were deposited via sequential pulsing of trimethylalumium (TMA) (97%, Sigma Aldrich) and tungsten hexafluoride (WF6) (99.8%, Sigma Aldrich). The deposition was performed at 200° C. in a hot-walled viscous flow ALD reactor. TMA and WF6 precursors were maintained at room temperature and ultrahigh purity N2 was used as a carrier gas with 300 sccm. The base pressure of the ALD reaction chamber was maintained at ˜1.0 Torr. TMA and WF6 were alternatively pulsed into the 15 sccm of N2 carrier flow with the following time sequence: 1 s WF6 dose—5 s purge—1 s TMA dose—5 s purge. An in situ QCM study was performed to study the nature of the deposition. The QCM measurements typically used longer N2 purge times of 10 s to allow the QCM signal to stabilize after each precursor exposure. The thicknesses of AlWxFy coating layers were determined by ex situ spectroscopic ellipsometry using a Cauchy model (Alpha-SE, J. A. Woollam Co.). The resistance of the film deposited on fused silica was determined by performing current-voltage (I-V) measurements using a four point probe measurement (Keithley Model 6487 current voltage source). X-ray photoelectron spectroscopy of AlWxFy films deposited on silicon was performed by Evans Analytical Group.
Transmission electron microscopy: Microstructure and crystallinity of AlWxFy films deposited on silicon wafer were analyzed using TEM. TEM analysis was performed by Evans Analytical Group (Sunnyvale, Calif.). Cross-section TEM samples were prepared using the in-situ focused ion beam (FIB) lift out technique on an FEI Strata Dual Beam FIB/SEM. The samples were capped with a protective layer of carbon prior to FIB milling, and were imaged with a FEI Tecnai TF-20 FEG/TEM operated at 200 kV in bright-field (BF) TEM mode, high-resolution (HR) TEM mode, and nano-beam diffraction (NBD) mode.
Electrochemical measurements: The LiCoO2 electrodes were prepared by mixing a slurry of commercial LiCoO2 powder (Sigma-Aldrich), Super-P carbon, and polyvinylidenedifluoride (PVDF) with a mass ratio of 84:8:8 in N-methyl pyrrolidone (NMP). The mixed slurry was caste on an aluminum foil current collector. AlWxFy films were deposited via ALD on the finished electrode laminates with thicknesses of ˜1 nm. 2032-type coin cells were assembled in an Ar-filled glovebox (water and oxygen≤1 ppm) with metallic lithium anodes. The electrolyte consisted of a 1.2M LiPF6 solution in ethylene carbonate and ethylmethyl carbonate (3:7 mixture). Charge-discharge measurements were recorded on a MACCOR potentiostat at room temperature under a current rate of 20 mA/g between 2.5 V and 4.4 V. Rate capability experiments were conducted with a constant charge rate of 20 mA/g and discharge rates of 20, 100, 200, and 400 mA/g.
X-ray photoelectron spectroscopy: The chemical state of aluminum before and after electrochemical cycling was analyzed by XPS (Sigma Probe: Thermo VG Scientific) with monochromatic Al Kα radiation (1486.6 eV). The cycled 2032 coin cells were disassembled and washed with dimethyl carbonate (DMC), and transferred directly to XPS to eliminate air-exposure during sample transport.
The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.
Elam, Jeffrey W., Mane, Anil U., Park, Joong Sun, Croy, Jason R.
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